This invention relates to a method and to an apparatus for coating board-shaped articles, especially printed circuit boards.
After they have been provided with conductors, printed circuit boards are coated with a protective layer. The protective layer is a preferably UV-hardenable photographically exposable plastics material, lacquer or the like. After drying of the protective layer, it is exposed in a suitable form and developed at the exposed sites. The conductors are then free at those sites and can be electrically contacted. In the case of printed circuit boards that are provided with conductors on both sides, both surfaces of the boards have to be coated without, when coating the second side, damaging the coating already applied to the first side. In order to be coated, the printed circuit boards have to be transported through a series of processing stations of a coating installation. There the previously cleaned printed circuit boards are first pre-heated and provided with a protective layer on one side. For that purpose, the printed circuit boards to be coated are transported through a coating station, preferably under a free-falling pouring curtain. When the first side has been coated, the printed circuit boards are transported to a vapour-removal and drying station in which the solvent is evaporated (removed by a current of air) and the coated surface of the printed circuit boards is dried. If the second side of the printed circuit boards is also to be coated, the boards are turned, transported under a pouring curtain of a coating station again and finally subjected to vapour removal and dried in a vapour-removal and drying station.
When the printed circuit boards are being transported from one processing station to the next, especially when the boards are being transported while the coating is still wet and in the case of printed circuit boards already coated on one side, care must be taken that the printed circuit board and the coating are not damaged or impaired. The transport of the printed circuit boards in the individual processing stations and from station to station is effected by means of various automatic conveyors. The trend to use ever thinner printed circuit board materials, however, gives rise to the problem that the printed circuit boards tend to sag while being transported, especially owing to the thermal treatment in the preheating station and in the vapour-removal and drying stations, and to touch the conveyors or transport means with their undersides. This is a particular problem when the printed circuit boards are to be coated on both sides and, in so doing, the still soft protective coating on the undersides of the printed circuit boards is damaged. In addition, the conveyors or transport means can also become soiled.
The prior art has accordingly proposed various transport apparatuses and holding devices for printed circuit boards in the various processing stations. U.S. Pat. No. 5,113,701 proposes that the printed circuit boards be inserted after coating into holding frames having prongs arranged in a V-shape. Those holding frames are fixed to a drying station and have a predetermined frame width. Outside the drying station, the printed circuit boards are transported on roller conveyors arranged in a V-shape. Although in the case of that proposed solution the printed circuit boards are touched and supported substantially only at their longitudinal edges, it may happen, especially in the case of very thin printed circuit boards of great length and width, that they nevertheless rest on the prongs of the holding frames or on the rollers of the roller conveyors. As a result, and owing to the fact that the holding frames and/or the roller conveyors have a fixed width, other printed circuit boards transported in the soiled frames may become contaminated. Furthermore, the proposed coating installation requires a separate centring station which aligns the printed circuit boards before they are transported under the pouring curtain in the coating station.
U.S. Pat. No. 4,926,789 describes holding devices for printed circuit boards that have clips which grip the printed circuit boards at several sites along their longitudinal edges. In the proposed coating installation the printed circuit boards are transferred together with the holding devices from one processing station to the next. Not until the exit of the coating installation is reached are the holding devices removed from the boards again and transported back to the entrance to the installation. The described installation is relatively complicated in terms of construction and requires separate feed lifts, conveyor belts and transport means for the holding devices. In addition, actuating devices for the holding clips of the holding devices have to be provided at the entrance and the exit of the coating installation. There is also a risk in the coating station that the holding devices will become soiled during transport under the pouring curtain.
U.S. Pat. No. 4,848,533 describes a printed circuit board transport apparatus that has two chains or the like circulating parallel to one another. The chains are connected by cross-pieces arranged at a distance from one another. Two holding devices constructed as clamps are mounted on each cross-piece so as to be displaceable relative to one another in a braked manner. In a board-receiving station, two pneumatically driven rams push the passing holding devices in pairs onto the longitudinal edges of the boards where they become clamped. Thus, at least two, but generally more than two, pairs of holding devices are clamped along the longitudinal edges of a printed circuit board, and the latter is transported along the transport path, held in that manner. In a board unloading station are provided two other rams which push the holding devices apart again, as a result of which the printed circuit boards are released again. In the case of the described transport apparatus, the printed circuit boards are held at their longitudinal edges by at least two pairs of holding devices, thus ensuring that the undersides of the boards do not come into contact with the transport chains or with the cross-pieces. The transport apparatus is, however, designed only for the horizontal transport of the printed circuit boards, for example under the pouring curtain. In order to transfer the boards, for example, to the drying station, the holding devices must first be removed again, and the boards must be loaded, for example, onto a roller conveyor by means of which they can then be moved into the holding devices of the drying station. The apparatus is relatively complicated in terms of construction. The many pairs of holding devices, which must be displaceable relative to one another on the cross-pieces, constitute a possible source of faults. For example, one or more cross-pieces could be soiled, which can greatly impair the displaceability of the holding devices with the result that reliable grasping of the printed circuit boards is no longer ensured.